The typical hard disk drive includes a head disk assembly (HDA) and a printed circuit board (PCB) attached to a disk drive base of the HDA. The head disk assembly includes at least one disk (such as a magnetic disk, magneto-optical disk, or optical disk), a spindle motor for rotating the disk, and a head stack assembly (HSA). The printed circuit board assembly includes electronics and firmware for controlling the rotation of the spindle motor and for controlling the position of the HSA, and for providing a data transfer channel between the disk drive and its host.
The head stack assembly typically includes an actuator, at least one head gimbal assembly (HGA), and a flex cable assembly. Each HGA includes a head for reading and writing data from and to the disk. In magnetic recording applications, the head typically includes an air bearing slider and a magnetic transducer that comprises a writer and a read element. The magnetic transducer's writer may be of a longitudinal or perpendicular design, and the read element of the magnetic transducer may be inductive or magnetoresistive. In optical and magneto-optical recording applications, the head may include a mirror and an objective lens for focusing laser light on to an adjacent disk surface.
During operation of the disk drive, the actuator must rotate to position the heads adjacent desired information tracks on the disk. The actuator includes a pivot bearing cartridge to facilitate such rotational positioning. One or more actuator arms extend from the actuator body. An actuator coil is supported by the actuator body opposite the actuator arms. The actuator coil is configured to interact with one or more fixed magnets in the HDA, typically a pair, to form a voice coil motor. The printed circuit board assembly provides and controls an electrical current that passes through the actuator coil and results in a torque being applied to the actuator. A crash stop is typically provided to limit rotation of the actuator in a given direction, and a latch is typically provided to prevent rotation of the actuator when the disk dive is not in use.
The spindle motor typically includes a rotor including one or more rotor magnets and a rotating hub on which disks mounted and clamped, and a stator. If more than one disk is mounted on the hub, the disks are typically separated by spacer rings that are mounted on the hub between the disks. Various coils of the stator are selectively energized to form an electromagnetic field that pulls/pushes on the rotor magnet(s), thereby rotating the hub. Rotation of the spindle motor hub results in rotation of the mounted disks.
Excessive imbalance of the disk mounting hub, disk clamp, disk(s), and spacer rings (if any), can cause undesirable disk drive vibrations and associated customer complaints. In extreme cases, such vibrations might even degrade the ability of the actuator to position the heads adjacent desired information tracks on the disk. Therefore, it is advantageous to balance the hub, clamp, disk(s), and spacer rings (if any) while or after they are assembled together. Adding, removing, or moving mass in a single plane normal to the axis or rotation of the hub can counteract a net radial imbalance force on the rotor (i.e. a net imbalance force that would tend to dynamically translate the axis of rotation). Such single plane balancing can be accomplished, for example, by a balancing ring of appropriate size and mass affixed to the top of the disk clamp.
However, balancing in at least two planes is required to counteract both a net radial imbalance force on the rotor and a net tilting moment acting upon the rotor (i.e. a moment that would tend to dynamically tilt the axis of rotation). Such dual plane balancing is often cumbersome and expensive because it requires adding, removing, or moving mass in a second plane that is not easily accessible from the top of the motor hub after assembly of the disk(s), clamp, and spacer rings (if any). If a second balancing ring were placed lower on the hub, there would be insufficient access from the top to conveniently move (i.e. rotate) the second ring after disk drive assembly, for balancing purposes. Moreover, attempting to remove mass deep within the hub after disk assembly is generally impractical because of the likelihood of unacceptable particle generation and/or component damage while removing sufficient mass (especially if the hub is fabricated from a low mass density metal such as aluminum). Therefore, most conventional attempts at dual plane balancing of disk drive rotors have relied upon adding selected masses at different depths within receiving holes pre-bored into the hub. Such methods, although effective, are costly and cumbersome in a high-volume manufacturing environment because of the necessity to track inventory and part numbers for various masses having different desired characteristics (e.g. different mass and/or different center of gravity offset from the bottom of the receiving hole).
Thus, there is a need in the art for an improved balancing structure for a disk drive motor that is suitable for use in a high-volume manufacturing environment.